1. Field of the Invention
The present invention relates to a positive temperature coefficient thermistor and more particularly to a laminate-type positive temperature coefficient thermistor having a greatly improved withstand voltage property.
2. Description of the Related Art
Generally, laminate-type positive temperature coefficient thermistors have the following structure (for example, see Japanese Unexamined Patent Application Publication No. 5-47508).
In particular, a laminate-type positive temperature coefficient thermistor includes a substantially rectangular laminate with a positive resistance-temperature coefficient. The laminate has a plurality of laminated thermistor layers, and first and second external electrodes formed on the outer surface, that is, on the first and second opposed end surfaces of the laminate.
Moreover, a plurality of first and second internal electrodes are uniformly formed on predetermined interfaces between the thermistor layers inside the laminate. The first and second internal electrodes are electrically connected to the first and second external electrodes. The first and second internal electrodes are alternately arranged in the lamination direction such that a portion of the first internal electrodes and a portion of the second internal electrodes overlap each other.
For positive temperature coefficient thermistors, it is necessary to have a sufficient withstand voltage property. Referring to the withstand voltage properties of the laminate-type positive temperature coefficient thermistors having the above-described structure, a breakdown occurs in the center of the laminate. Specifically, in some cases, the breakdown occurs in the center in the lamination direction of the portion where the first and second internal electrodes are arranged and in the center in a direction that is substantially perpendicular to the lamination direction of the portion of the laminate where the first and second internal electrode overlap each other. The breakdown arises due to the heat-dissolution of a semiconductor ceramic constituting the thermistor layers. In particular, for example, the laminate is heated when a voltage is applied to the laminate-type positive temperature coefficient thermistor for evaluation of the withstand voltage property. The generated heat is stored in the center of the laminate. Thus, the center of the laminate becomes a hot-spot. As a result, the “explosion of heat” occurs, so that the semiconductor ceramic constituting the thermistor layers is heat-dissolved. Probably, the heat-dissolution causes the above-described breakdown in the center of the laminate.